X-ray cargo inspection systems typically use an X-ray fan beam generated by a pulsed high-energy X-ray source, such as a linear accelerator (linac) or a betatron. The highest available pulse rates from these sources limit the line frequency of the imaging system and thus the maximum scan speed for a given line resolution. Linear accelerators are available with pulse rates up to 1000 pulses per second (pps). At that rate an object with a speed of 60 km/h moves 16.7 mm per pulse. In order to achieve a typical 4 mm vertical line pair resolution, four image lines must to be acquired simultaneously. Employing multiple sources with multiple detector arrays, however, is a costly proposition.
FIG. 1 depicts a cargo inspection system employing an x-ray transmission technique. A fan-shaped beam 12 of penetrating radiation, emitted by a source 14, is detected by elements of a detector array 16 distal to a target object, here truck 10, in order to produce images of the target object. Particular contents of the object may be discriminated and characterized on the basis of the transmission of penetrating radiation through the object and its detection by detector array 16 and its individual detector modules 18. (As used herein, the term “detector module” refers to a detector element in conjunction with its associated preprocessing electronics.) Signals from each of the detector modules, suitably pre-processed, provide inputs to processor 19, where material characteristics are computed.
Information (such as mass absorption coefficient, effective atomic number Zeff, electron density, etc.) regarding the material composition of the contents of objects may be obtained on the basis of the interaction of X-rays with the material, and, more particularly, by illuminating the material with X-ray beams having energy spectra with more than one distinct energy endpoint (peak energy), or by employing energy discriminating detectors. Dual energy methods of material discrimination are widely used in X-ray inspection systems for security control of hand luggage in customs and other security checkpoints. Dual energy inspection is discussed in the following references, for example, which are incorporated herein by reference:    U.S. Pat. No. 5,524,133, Neale et al., “Material Identification using X-Rays” (1996) (hereinafter, “Neale '133”)    U.S. Pat. No. 7,257,188, Bjorkholm, “Dual Energy Scanning of Contents of an Object” (2005)    U.S. Pat. No. 6,069,936, Bjorkholm, “Material Discrimination using Single-Energy X-Ray Imaging System” (2000)
A multi-view x-ray inspection system is disclosed in US Published Patent Application US 2011/0206179 (“Bendahan”), incorporated herein by reference, which suggests rapidly steering a single electron beam to a sequence of x-ray radiation-producing targets, and shows an embodiment in which a beam appears to be detected by multiple parallel detector arrays, although this embodiment is not described in detail.